In general, a cordless electric tool uses a secondary battery pack that is rechargeable with a charging device as a power supply. A lithium ion (Li-ion) battery cell is commonly used to form the secondary battery pack. A charging device for a Li-ion type battery pack generally charges the battery pack with a constant charging current and at a constant charging voltage. In particular, in order to avoid reverse charging for each battery cell, the charging device charges the battery pack with the constant charging current until the battery voltage reaches a predetermined value, and then charges at the constant charging voltage until the current passing through the battery pack reduces under a predetermined value due to full charging.
There are two types of battery pack to be charged by the charging device: a first type of battery pack and a second type of battery pack. The first type of battery pack is a 4S1P type of battery pack having a nominal voltage of 14.4 V, in which four battery cells are connected in series. The second type of battery pack is a 4S2P type of battery pack having a nominal voltage of 14.4 V, in which a pair of battery cells is connected in parallel and four pairs of parallel-connected battery cells are connected in series.
If one charging device selectively charges the above two different types of battery packs and flows the same amount of charging current through each battery pack, the amount of current passing through each cell of the 4S1P type is twice as much as the amount of current passing through each cell of the 4S2P type. This phenomenon may result in shortening the lifespan of the 4S1P type, compared with the lifespan of the 4S2P type.
The charging voltage also influences the lifespan of the battery pack. If each battery cell is charged at a lower charging voltage, the lifespan of the battery pack is expected to be extended.
Further, an amount of discharging current from each cell of the 4S1P type is generally more than an amount of discharging current from each cell of the 4S2P type. Even if each type of battery pack is charged at the same voltage, the lifespan of the 4S1P type is shorter than the lifespan of the 4S2P type because of an amount of discharging current per cell.
An object of the present invention is to provide a charging device which charges a battery pack with a proper current and at a proper voltage depending on the type of battery pack to avoid affecting the lifespan of the battery pack.